Mononuclear phagocytes MPs play crucial roles in the initiation of innate and adaptive immune responses and in the maintenance of tissue homeostasis. Although MPs share several phenotypic and functional characteristics, it has recently become clear that dendritic cells (DC), macrophages (Mac) and monocytes (Mo) are not homogeneous populations and instead they represent developmentally and functionally distinct populations that differentially regulate T cell function. MPs are major components of the tumor microenvironment where they play a dual role inducing adaptive anti-tumor responses but also sustaining immune evasion, tumor progression, and metastasis formation. Despite major advances in the identification of the MPs developmental pathways and their transcriptional regulation, the individual contribution of these distinct cell subsets to the induction and resolution of immunity against invading pathogens, or to anti-tumor responses or immune evasion, as well as the environmental signals involved in their regulation remain unclear. In this project we will use murine experimental tumor models to investigate the mechanisms regulating MP differentiation and function, with particular emphasis on the role of the commensal microbiota. Local and systemic inflammation modulates cancer susceptibility (e.g. in obesity), cancer progression, response to therapy, and co-morbidity (e.g. cancer cachexia/anorexia). The microbiota influences both immune and metabolic function beyond the gut, including peripheral innate cell responses, autoimmunity and response to viral infections. In recent studies we have shown that gut commensals control the response of subcutaneous tumors to immunotherapy and chemotherapy by modulating tumor infiltrating MP function (Science 2013, 342:967-970). This study demonstrates the novel finding that and intact gut microbiota is needed for optimal response to cancer therapy and underscores the potential to improve cancer treatment by manipulating the gut microbiota. However, the exact molecular mechanisms by which commensal bacteria modulate systemic inflammation are still unknown. We use several approaches to address this question performing parallel studies in cancer and infection models. We utilize the germ-free (GF) facility at NCI-Frederick to compare conventionally reared, GF animals, or animals treated with different antibiotics or given different diets to modulate the microbiota composition either under steady state or different inflammatory settings. We have characterized the innate myeloid and lymphoid infiltrate in several tumor models in the presence or absence of intact microbiota. Our results showed that in the absence of an intact microbiota there is a skewing of myeloid cell differentiation towards a pro-tumoral phenotype. In addition, the composition of the innate lymphoid cell populations is significantly altered compromising the anti-tumoral response. We have also observed that while the microbiota affects the myeloid cell compartment in the bone marrow, there are specific changes that occur in the tumor that are not observed in the bone marrow, blood or peripheral lymphoid tissues. We are currently addressing the underlying mechanisms with particular emphasis on the role of bacteria-derived metabolites. In a different set of studies, we are addressing the role of different myeloid cell populations in the response to cancer therapy versus their pro-tumorigenic function in untreated tumors. In parallel studies, we are looking at the impact of the gut microbiota on the monocyte/macrophage cellular dynamics in the context of infection. We have shown that chronic infection with a strong Th1 inducing pathogen leads to systemic disruption of monocyte/macrophage homeostasis, we are currently investigating the physiological consequences and whether altering the commensal microbiota composition would either contribute or prevent such disruption.